The marine biogeochemistry of selenium: A re‐evaluation1

Abstract

Vertical and horizontal profiles from the North and South Pacific Oceans demonstrate the existence of three species of dissolved selenium: selenite, selenate, and organic selenide (operationally defined). In surface waters, organic selenide makes up about 80% of the total dissolved selenium, selenite concentrations are uniformly low, and selenate concentrations rise with increased vertical mixing. The organic selenide maximum (thought to consist of seleno-amino acids in peptides) coincides with the maxima of primary productivity, pigments, bioluminescence, and dissolved free amino acids. Deep ocean waters are enriched in selenite and selenate, while organic selenide is nondetectable. In suboxic waters of the tropical northeastern Pacific, organic selenide concentrations rise, while selenite values decrease. The downward flux of particulate selenium generally decreases with depth, and fluxing particulate selenium is found to be primarily in the (-2) oxidation state. These data allow a re-evaluation of the internal biogeochemical cycle of selenium. This cycle includes selective uptake, reductive incorporation, particulate transport, a multistep regeneration, and kinetic stabilization of thermodynamically unstable species.

Emerging Opportunities in Management of Selenium Contamination

“…Biological processes are difficult to predict from environmental characteristics, so conventional speciation modeling is problematic for Se. On the other hand, Se is one of the few elements for which the different species can be directly analyzed at environmental concentrations (14,15). These data show that geologic and anthropogenic sources often release mostly SeO 4 2-(8), which is not reactive with particle surfaces, although some types of bacteria convert SeO 4 2-to elemental Se in sediments ( Figure 1; (16)).…”

“…Unlike most trace elements, the distribution of Se among dissolved species cannot be predicted from thermodynamics alone. Biological (kinetically driven) processes are just as important as geochemical processes in determining the forms of Se that are present (14). Biological processes are difficult to predict from environmental characteristics, so conventional speciation modeling is problematic for Se.…”

“…Downstream in the delta of the San Joaquin River, SeO 3 2-, organo-selenide, and SeO 4 2-are in equal abundance (15). In the Pacific Ocean the metalloid is nearly 100% SeO 3 2-and organo-selenide (14). This unidirectional build-up of potentially reactive forms, especially in environments where water residence times are extended (e.g., wetlands, estuaries) is a key factor in the ecological risks posed by Se.…”

Selenium (IV), selenium (VI) and organic selenium in Lake Biwa, the Yodo River and Osaka Bay.

“…The concentration of organic selenium increased gradually from 0.40 nM at the surface to 0.65 nM at 5 m, then decreased gradually towards the bottom. The concentrations of Bruland (1984) reported that the distribu tion and speciation of dissolved selenium in the eastern tropical North Pacific Ocean, the concen trations of selenium (IV), selenium (VI) and organic selenium ranged from 0.02 to 1.03, from 0.13 to 1.48 and from 0.01 to 1.06 nM, respec tively. The concentrations of the three selenium species at St. 15 were lower than those in the North Pacific Ocean.…”

“…Several in vestigators have reported that selenium. is pre sent in three chemical forms, such as selenium (IV), selenium (VI) and organic selenium (Sugimura and Suzuki, 1985;Takayanagi and Wong, 1985;Cutter and Bruland, 1984;Nakaguchi et al, 1985;Aono et al, 1991). The vertical distribution of selenium (IV) was similar to those of nitrate + nitrite and phosphate which is contained in the phytoplankton cell and thus the behavior of selenite in sea water might be related with marine phytoplankton activities (Aono et al, 1991).…”